1. Field of the Invention
This invention relates generally to nickel-hydrogen storage cells, and, more particularly, to the use of such cells in which the hydrogen gas is stored as a metal hydride at a location remote from the storage cell and can be selectively dispensed to the storage cell.
2. Description of the Prior Art
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. A familiar example of the rechargeable cell is the lead-acid cell used in automobiles. Another type of cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications.
The nickel-hydrogen cell includes a series of active plate sets which store a charge electrochemically and later deliver that charge as a useful current, packaged within a pressure vessel that contains the electrolyte, the plate sets, and the hydrogen gas that is an essential active component of the cell. A nickel-hydrogen storage cell delivers current at about 1.3 volts, and a number of the cells are usually connected in series to produce current at the voltage required by the systems of the spacecraft.
A nickel-hydrogen cell as typically used in a satellite is periodically charged by electrical current produced by solar panels on the spacecraft when the satellite is in sunlight, and then later discharged to supply electrical power, when the spacecraft is in shadow or peak electrical power is demanded.
Each cell has a positive (during discharge), nickel-containing electrode, consistently designated as "cathode" herein, spaced from a hydrogen-containing negative (during discharge) electrode consistently designated as "anode" herein. The electrodes generally have the form of spaced plates separated by a porous inert sheet, such as zirconium oxide cloth, asbestos, polypropylene or nylon, which acts as a separator matrix for electrolyte extending between the two electrodes. The separator matrix sheet is sufficiently thick to prevent short circuit contact between the electrodes and holds a sufficient quantity of electrolyte for desired cell performance. The electrolyte is an alkaline medium, preferably an aqueous solution of alkali metal hydroxide, more particularly thirty percent potassium hydroxide solution. The hydrogen-containing electrode is a plastic bonded, metal powder or carbon or catalyzed carbon plate. The metal is preferable platinum, but may comprise other materials which will catalyze hydrogen oxidation reactions in aqueous electrolyte media and is backed by a plastic, preferably tetrafluoroethylene (e.g., Dupont's Teflon brand materials), mesh element which accommodates gas diffusion. The cathode material is a nickel-oxy-hydroxide. Electrode pairs are generally arrayed with their cathodes back to back. External contact to the electrodes is generally made by nickel. Hydrogen within the pressure vessel, generally maintained at a superatmospheric pressure of 20-100 atmospheres, diffuses through the gas diffusion mesh of Teflon or the like to reach the catalytic anode where the discharge mode anode reaction,
(I) 1/2 H.sub.2 +OH.fwdarw.H.sub.2 O+e.sup.- occurs, in balance with the corresponding cathode reaction, PA1 (II) NiOOH+H.sub.2 O+e.sup.- .fwdarw.Ni(OH).sub.2 +OH.sup.- providing an overall discharge reaction, PA1 (III) NiOOH+1/2 H.sub.2 .fwdarw.Ni(OH).sub.2,
The reverse of such reactions occur on charging.
Safety problems with batteries are in part due to the mixing of the active positive and negative materials within a single container. In this view gasoline is fundamentally less dangerous than a battery as oxygen is not present in large amounts inside the tank. The intimate mixing of positive and negative materials inside a battery container is in the view of conventional wisdom required to provide adequate power density and to maximize volumetric energy density. As to the former, batteries with thicker electrodes are viewed as safer but must operate at higher current densities to produce an equivalent power. As to the latter, the more closely packed the battery materials are the higher will be the volumetric energy density.
One exception to the above approach is found in flow batteries such as zinc bromine and zinc chloride. In this case the positive reactants must be stored external to the battery stack. They are introduced into the battery stack by means of mechanically complex pumps. Typical of this general construction are the U.S. Pat. Nos. to Maricle, 4,129,683 and 4,128,701 in which hydrogen and chlorine are pumped from storage tanks to their respective chambers of a regeneration fuel cell.
Various disclosures are known of nickel hydrogen electrical storage cells contained within suitable pressure vessels. Such disclosures include the U.S. Pat. Nos. to Lim et al., 4,820,597; to van Ommering et al., 4,565,749; to Holleck, 4,127,703; and to Tsenter et al., 3,699,744.
In U.S. Pat. No. 3,850,694 to Dunlop et al., a nickel hydrogen fuel cell containing lanthium nickel is initially charged with hydrogen gas to thereby form lanthium nickel hydride after which the filling tube is permanently closed off. In this manner, hydrogen is stored as a reduced compound rather than as a gas at higher pressures.
Another pertinent patent, U.S. Pat. No. 5,082,754 to Jones, discloses a metal oxide-hydrogen battery which comprises an outer cylindrical shell and a battery cell disposed centrally within the shell. Additionally, a pair of sealed spherical tanks are located in the ends of the shell on either side of the cell and each spherical tank contains pressurized hydrogen. An opening provides communication between the cylindrical shell and each of the spherical tanks and a remotely controlled valve acts to open and close the openings. Opening of the valve permits hydrogen gas to contact the cell to generate an electron current in an exterior circuit. During recharging of the cell, hydrogen generated in the cell passes into the spherical tanks and is captured therein by closing of the valves so as to minimize self-discharge of the battery cell during standby periods. In short, the Jones patent teaches a hydrogen storage vessel as a structural part of a battery.
It was in light of the prior art as just described that the present invention was conceived and has now been reduced to practice.